1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
Large flat panel displays (FPD's) are being designed which require the exposure of very large thin glass substrates. For example, glass substrates with a thickness of less than 1 mm and edge dimensions typically of 1.85×2.2 meters. On each glass substrate, several panels (e.g. 4, 6, or 9) may be defined, with each panel corresponding to a single product such as a computer monitor screen or a TV screen. With such large substrates, it has been proposed to mount the substrate on a displaceable table and to then displace the table using, for example, linear motors. The table is thus functionally equivalent to a chuck, and must be moved with precision to ensure that the substrate supported on the chuck is in the appropriate position relative to a projection beam. With such an arrangement, a combined mass of the moving components (e.g., the substrate and the chuck) is larger than the mass of the substrate itself. This results in high inertial forces, which are generally undesirable in apparatus requiring high positional precision.
It has also been proposed, in the context of exposing large substrates for FPD's, to rely upon a stationary optical column (e.g., components that generate the projection beam) and a substrate displacement system that moves the substrate along a simple linear path beneath optical column, such that the projection beam is scanned across the substrate. Thus, a width of the projection beam must be equal to a width of a target portion of the substrate. This can be achieved by appropriate design of optical column, but it is nonetheless necessary to very accurately control the position of the substrate along its direction of displacement relative to optical column.
It is known to support substrates, such as large glass panels, on, for example, a pressurized air bearing, such that the panel is readily displaceable on that bearing relative to an upper surface of a static support table. Such a “floating” substrate support system could then rely upon any appropriate arrangement to achieve displacement of the substrate. This could be, for example, by applying a mechanical force directly to an edge of the substrate, by directing jets of air against the substrate such that a net force results in the desired direction of displacement, or by applying force using a magnetic arrangement coupled to the substrate. Unfortunately, in the context of lithographic apparatus, where positional accuracy is fundamental, it is very difficult to use such a displacement system itself to indicate substrate position. For example, where a lithographic apparatus is provided with a displaceable chuck, alignment marks (often referred to as “encoder gratings”) can be provided on the chuck itself. From those alignment marks, and a predefined known relative position, as between the chuck and the substrate, the position of the substrate can be accurately determined. Where the substrate is “floating” relative to the underlying table, however, the position of the substrate itself can only be determined by reference to alignment marks or other physical characteristics of the substrate itself. Such alignment marks must be provided on the substrate before the substrate is presented to the lithographic apparatus, and therefore an extra processing step, and possibly an additional lithographic apparatus, must be provided simply for the provision of alignment marks on the substrate.
A further problem with controlling the position of a large area thin substrate on a floating bearing is that of accurately controlling the force applied to the substrate, so as to achieve uniform and predictable displacement of the substrate. The simple application of a mechanical force to one edge of the substrate is problematic given its thickness relative to its area, displacement relying upon air pressure is difficult to control, and there is nothing on the substrate to enable its magnetic coupling to an appropriate magnetic drive.
Therefore, what is needed is a lithographic system and method that allow for accurate positional measurement and/or movement of a large work piece during patterning of the work piece.